Sadaaki Iwanaga and Shun-ichiro Kawabata

Department of Biology, Faculty of Science, Kyushu University, Fukuoka 812-8581, Japan

Received 7/13/98 Accepted 8/21/98

2. INTRODUCTION

Horseshoe crab are often called living fossils, because they belong to the subclass Xiphosura which spans over 500 million years of evolution (1). Today, only four species are known in various regions of the world and classified into two subfamilies, Limulinae and Tachypleinae (2, 3). Limulinae consists of one genus and one species, Limulus (L.) polyphemus, which inhabits the east coast of the United States. Tachypleinae includes two genera and three species, Tachypleus (T.) tridentatus, T. gigas, and Carcinoscorpius (C.) rotundicauda, which inhabit Thailand, India, Malaysia, South China, and Japan. These classifications have been based on morphological and/or anatomical investigations, and there was little known on the molecular and genetic level, viz. the phylogenetic relationship of the four living species. However, recent biochemical studies on the defense molecules, which are closely associated with the innate immunity of horseshoe crabs (4-11), provided much information, not only about phylogeny, but also about the molecular evolution of these components. During the past two decades, at least more than fifty defense molecules in hemolymph of horseshoe crabs have been identified and biochemically characterized. Here, we will focus on our recent progress with respect to the phylogenetic relationship and evolution of these major molecules.